Solving the Tully Monster’s Cold Case

Since it was first uncovered more than a half a century ago, this kooky-looking creature known as the “Tully monster” has puzzled paleontologists who, frankly, could not make heads, tails or claws of its fossilized remains.

The creature was named after Francis Tully, the amateur who discovered it in 1958 in the Mazon Creek in Illinois. The state has designated the monster as its official fossil.

Some thought the 300-million-year-old creature was a mollusk, like a snail. Others assumed it was an arthropod like an insect or crab. And others believed it was some sort of worm.

Now, a team of researchers from Yale University say they have figured out the monster’s identity: It’s a vertebrate most closely related to the lamprey, an underwater bloodsucker.

They published their findings on Wednesday in Nature.

To come to their conclusion, team members first pored over 1,200 Tully monster specimens from museums. They closely examined the creature’s features, like its torpedo-shaped body and triangular tail, the proboscis that looks like an elephant’s trunk with sharp teeth, and the eyes on the side of its head, which resemble a hammerhead, but are similar to eye stalks found in crabs and insects.

“The frustrating thing is that these morphological features are not typical of any group,” said Victoria McCoy, a paleontologist and lead author on the paper. “But they do not rule out any group very easily.”

The clue that led them to closing the cold case was a lightly colored structure scientists had previously identified as the creature’s gut. Only it wasn’t a gut.

“We discovered that this feature was the notochord, the primitive backbone,” said Dr. McCoy.

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A 300-million-year-old Tully Monster fossil.Credit...Nicole Karpus

Most guts in the fossilized record are dark and appear three-dimensional. But the Tully monster’s structure was light and appeared two-dimensional.

“It didn’t make sense to us that there would be this one animal that would fossilize its gut completely differently,” she said.

After finding that the creature had a primitive backbone, they could classify it as a chordate, which is a family of species that includes all vertebrates. Then they had to narrow down the type of chordate to which it was most similar. By further examining the notochord, the fossil sleuths noticed that the structure curved down as it went through the creature’s tail.

In animals like sharks, the notochord curves up into the top fin of the tail, and in some fish it goes through the middle of the tail. But in lampreys the notochord curves down.

“There was no big ‘Aha!’ moment that pointed to the lamprey. But put together, the strongest evidence was that it could be a lamprey,” Dr. McCoy said. “The coolest thing is finding out that as weird as it looks, it is part of a familiar group of animals.”